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FIGURE 15 Diacylglycerol (DAG), formed from phosphatidylinositol 4,5 bisphosphate (PIP2) by the action of phospholipase C, may be cleaved by DAG lipase to release arachidonate, the precursor of the prostaglandins and leukotrienes. Arachidonate is also released from other membrane phospholipids (e.g., phosphatidylcholine) by the action of the enzyme phospholipase A2.

the effectors of responses to other G-protein receptors. In particular, it appears that the fiy subunit complex may activate potassium channels, and that a subunits may interact with calcium channels. The exact nature of the interaction of these proteins with ion channel proteins is not yet known. Because some membrane channel proteins are substrates for PKA and perhaps PKC, G-proteins also affect channels indirectly. Altering ion channel activity by the actions of G-protein-dependent agonists frequently results in modulating the responsiveness of target cells to the actions of other agonists.

Complexities of G-Protein-Related Transduction

We have described only a very limited set of generic mechanisms by which hundreds or perhaps thousands of diverse agonists arising in the external environment (e.g., light, odorants) and internal environment, including neurotransmitters and hormones, modify the behavior of their target cells through G-protein receptors. At first glance, there seem to be too few postreceptor mechanisms to accommodate the vast array of signals that impinge on cells. Actually, enormous diversity and complexity of responding systems can be achieved through selective, cell-specific expression of specific isoforms of almost every component of the response systems described above. To date, 16 genes for the G-protein a subunits have been isolated. Including alternately spliced products, mammals express at least 20 different a subunits, each endowed with unique properties that shape its activity. Considering that 5 different p and 7 different y subunits have also been described, the number of possible heterotrimeric G-proteins is potentially quite large. Eight different genes code for isoforms of adenylyl cyclase that are stimulated or inhibited by different isoforms of Ga and that have different sensitivities to phosphorylation (and hence modulation) by PKA, PKC, or CAM-kinase. Cyclic AMP can be degraded by phosphodiesterases belonging to four different families, one of which is activated by calcium/calmodulin and another by cyclic GMP. There are 2 major classes of PKA and at least 12 isoforms. Of the 9 isoforms of PKC, only 4 are considered to be conventional and require calcium for activation. Other forms are calcium independent, and all 9 are down-regulated by prolonged stimulation.

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